Fix T93106: Cycles SSS not working with normals pointing inside

3 files changed, 18 insertions, 9 deletions

diff --git a/intern/cycles/kernel/integrator/subsurface.h b/intern/cycles/kernel/integrator/subsurface.h
index 9010318257f..59b0cd2596c 100644
--- a/intern/cycles/kernel/integrator/subsurface.h
+++ b/intern/cycles/kernel/integrator/subsurface.h
@@ -71,6 +71,10 @@ ccl_device int subsurface_bounce(KernelGlobals kg,
   }
 #  endif
 
+  if (sd->flag & SD_BACKFACING) {
+    path_flag |= PATH_RAY_SUBSURFACE_BACKFACING;
+  }
+
   INTEGRATOR_STATE_WRITE(state, path, throughput) *= weight;
   INTEGRATOR_STATE_WRITE(state, path, flag) = path_flag;
 
diff --git a/intern/cycles/kernel/integrator/subsurface_disk.h b/intern/cycles/kernel/integrator/subsurface_disk.h
index 6146b8c41fc..22327268e02 100644
--- a/intern/cycles/kernel/integrator/subsurface_disk.h
+++ b/intern/cycles/kernel/integrator/subsurface_disk.h
@@ -47,6 +47,7 @@ ccl_device_inline bool subsurface_disk(KernelGlobals kg,
   const float time = INTEGRATOR_STATE(state, ray, time);
   const float3 Ng = INTEGRATOR_STATE(state, subsurface, Ng);
   const int object = INTEGRATOR_STATE(state, isect, object);
+  const uint32_t path_flag = INTEGRATOR_STATE(state, path, flag);
 
   /* Read subsurface scattering parameters. */
   const float3 radius = INTEGRATOR_STATE(state, subsurface, radius);
@@ -123,6 +124,9 @@ ccl_device_inline bool subsurface_disk(KernelGlobals kg,
     const int object = ss_isect.hits[hit].object;
     const int object_flag = kernel_tex_fetch(__object_flag, object);
     float3 hit_Ng = ss_isect.Ng[hit];
+    if (path_flag & PATH_RAY_SUBSURFACE_BACKFACING) {
+      hit_Ng = -hit_Ng;
+    }
     if (object_flag & SD_OBJECT_NEGATIVE_SCALE_APPLIED) {
       hit_Ng = -hit_Ng;
     }
diff --git a/intern/cycles/kernel/types.h b/intern/cycles/kernel/types.h
index 01700add22b..cae514d4dbd 100644
--- a/intern/cycles/kernel/types.h
+++ b/intern/cycles/kernel/types.h
@@ -279,33 +279,34 @@ enum PathRayFlag {
   PATH_RAY_SUBSURFACE_RANDOM_WALK = (1U << 20U),
   PATH_RAY_SUBSURFACE_DISK = (1U << 21U),
   PATH_RAY_SUBSURFACE_USE_FRESNEL = (1U << 22U),
+  PATH_RAY_SUBSURFACE_BACKFACING = (1U << 23U),
   PATH_RAY_SUBSURFACE = (PATH_RAY_SUBSURFACE_RANDOM_WALK | PATH_RAY_SUBSURFACE_DISK |
-                         PATH_RAY_SUBSURFACE_USE_FRESNEL),
+                         PATH_RAY_SUBSURFACE_USE_FRESNEL | PATH_RAY_SUBSURFACE_BACKFACING),
 
   /* Contribute to denoising features. */
-  PATH_RAY_DENOISING_FEATURES = (1U << 23U),
+  PATH_RAY_DENOISING_FEATURES = (1U << 24U),
 
   /* Render pass categories. */
-  PATH_RAY_SURFACE_PASS = (1U << 24U),
-  PATH_RAY_VOLUME_PASS = (1U << 25U),
+  PATH_RAY_SURFACE_PASS = (1U << 25U),
+  PATH_RAY_VOLUME_PASS = (1U << 26U),
   PATH_RAY_ANY_PASS = (PATH_RAY_SURFACE_PASS | PATH_RAY_VOLUME_PASS),
 
   /* Shadow ray is for a light or surface, or AO. */
-  PATH_RAY_SHADOW_FOR_LIGHT = (1U << 26U),
-  PATH_RAY_SHADOW_FOR_AO = (1U << 27U),
+  PATH_RAY_SHADOW_FOR_LIGHT = (1U << 27U),
+  PATH_RAY_SHADOW_FOR_AO = (1U << 28U),
 
   /* A shadow catcher object was hit and the path was split into two. */
-  PATH_RAY_SHADOW_CATCHER_HIT = (1U << 28U),
+  PATH_RAY_SHADOW_CATCHER_HIT = (1U << 29U),
 
   /* A shadow catcher object was hit and this path traces only shadow catchers, writing them into
    * their dedicated pass for later division.
    *
    * NOTE: Is not covered with `PATH_RAY_ANY_PASS` because shadow catcher does special handling
    * which is separate from the light passes. */
-  PATH_RAY_SHADOW_CATCHER_PASS = (1U << 29U),
+  PATH_RAY_SHADOW_CATCHER_PASS = (1U << 30U),
 
   /* Path is evaluating background for an approximate shadow catcher with non-transparent film. */
-  PATH_RAY_SHADOW_CATCHER_BACKGROUND = (1U << 30U),
+  PATH_RAY_SHADOW_CATCHER_BACKGROUND = (1U << 31U),
 };
 
 /* Configure ray visibility bits for rays and objects respectively,